Light valves having an electro-optically active element comprising a liquid crystal composite have been used in displays (directly driven, passive matrix, and active matrix addressed), windows, and privacy panels. In a liquid crystal composite, plural volumes or droplets of a liquid crystal material are dispersed, encapsulated, embedded, or otherwise contained within a matrix material such as a polymer. Exemplary disclosures include Fergason, U.S. Pat. Nos. 4,435,047; West et al., 4,685,771; Pearlman, 4,992,201; and Dainippon Ink, EP 0,313,053, the disclosures of which are incorporated herein for all purposes.
The liquid crystal composite is disposed between electrodes, at least one of the electrodes typically being patterned to form a matrix. The electrodes are supported by substrates. When voltage is applied to a pair of electrodes, an electric field is created and the liquid crystal located between the electrodes will become transmissive. In this optical state incident light is transmitted through the composite. When the voltage to the pair of electrodes is switched off, the electric field no longer exists and the liquid crystal composite between the electrodes changes its optical state to one in which incident light is substantially scattered and/or absorbed. In this state the material will typically be opaque with a frosty appearance if scattering is predominant or dark gray if absorption is predominant. By individually controlling the voltage applied to each pair of electrodes in an electrode matrix, a graphical image may be generated. The electrode matrix can be transparent or reflective and is typically a matrix of thin film transistors (TFT), MOS transistors, MIM diodes, or crossed patterned electrodes. The graphical image can be viewed directly, projected onto a viewing screen, or viewed as a virtual image on the eye. By combining red, green, and blue images, either via sequential illumination, for example, using field-sequential color with red, green, and blue light or via dedicated red, green, and blue pixels, a colored image may be formed.
Regardless of the exact nature of the liquid crystal composite or of the intended application (e.g., reflective versus transmissive, direct viewing versus projection versus virtual image viewing, normal mode versus reverse mode, etc.), a high contrast display is desirable since it provides improved image definition and quality. A number of different displays have been designed in response to this need. For example, in U.S. Pat. No. 4,732,456 a display with enhanced contrast is disclosed in which a lens or reflector means is used to direct light transmitted by the display onto a light absorbing target.
U.S. Pat. No. 5,076,668 discloses a system for achieving a high brightness display with excellent contrast qualities. The disclosed system utilizes a gain reflector disposed behind a display medium comprised of a liquid crystal material. The gain reflector is preferably an offset gain reflector, thus angularly offsetting the specular reflection from the reflected gain.
Although there are various methods of achieving a high contrast liquid crystal display, typically additional elements are required, thus adding to the overall complexity and cost of the display panel. Therefore an improved high contrast liquid crystal display that is simple to manufacture is desired.